STUDY: STATE RULES HAVE CUT BLACK CARBON

State diesel rules aimed at improving public health have also reduced levels of black carbon, a potent contributor to climate change, according to a new report commissioned by the California Air Resources Board.

The study was led by Veerabhadran Ramanathan, a professor of climate science at the Scripps Institution of Oceanography in La Jolla who collaborated with the Department of Energy’s Lawrence Berkeley National Laboratory and the Pacific Northwest National Laboratory in Richland, Wash.

California has targeted diesel pollution for decades, including a series of measures starting in 2000. One of the most controversial diesel mandates was approved in 2008, focusing on diesel-powered trucks and buses, as part of a sweeping set of regulations designed to meet federal air-quality standards and cut down on cases of respiratory illness and premature death. Supporters have hailed it as a major tool for battling asthma and cancer, while opponents have said it’s an overly aggressive measure that has cost many jobs.

Ramanathan and his colleagues said their conservative estimate shows the diesel requirements may slow climate change by curbing the equivalent of 21 million metric tons of carbon dioxide annually. That’s about the same as taking more than 4 million vehicles off the road every year, the report concluded.

“Diesel is a toxin and is high on our list of dangerous emissions, so we’re really getting a global warming benefit as well as a public health benefit as a result of reducing diesel exhaust,” said San Diego County Supervisor Ron Roberts, who has also served on the Air Resources Board during its years of discussion about diesel pollution.

There’s general consensus among scientists that the Earth’s climate is warming and that fossil-fuel combustion is likely the leading cause. While carbon dioxide is the main contributor, black carbon produced by diesel exhaust and other emissions is No. 2, Ramanathan said.

His globe-trotting work, from China to California, has examined short-lived greenhouse compounds including black carbon, which lasts two weeks in the atmosphere, compared to carbon dioxide, which can persist for centuries. So cutting diesel emissions can swiftly reduce the rate of climate change, he said. “If this can be replicated worldwide, we can reduce projected global warming by as much as 15 percent over the next 30 years,” Ramanathan said.

The Air Resources Board commissioned the report through a competitive-bid process, selecting Ramanathan and his collaborators.

To conduct the study, Ramanathan said, the scientists examined decades of black-carbon measurements. “From the 1980s, black carbon had come down 50 percent,” he said. “Since the 1960s, the amount of soot we put out came down by 90 percent.”

Diesel use rose three- to four-fold during the same period, he said, but improved fuel formulations and engine filters slashed the amount of black carbon emitted.

Early on, concentrations of black carbon dropped in California because of state rules controlling tailpipe emissions and the burning of trash and coal, said Melanie Turner, a spokeswoman for the board. Upcoming phases will address diesel exhaust from agricultural equipment, she said.